1. Field Of The Invention
This invention relates generally to a high electron mobility transistor and, more particularly, to a pseudomorphic high electron mobility transistor incorporating a partially relaxed InGaAs channel having a thickness greater than the critical thickness.
2. Discussion of the Related Art
High electron mobility transistors (HEMTs), well known in the art, are used in various low-noise and power microwave applications where relatively high device output power, power added efficiency and noise performance are critical. Specific applications for HEMTs include Q, V and W band microwave power amplifiers for use in commercial and military radar systems, communication systems, etc. HEMTs can be effectively integrated into monolithic microwave integrated circuits and monolithic millimeter-wave integrated circuits (MMICs) including phased arrays for radiating at high power levels.
HEMTs are generally one of two types. These two types include regular HEMTs and pseudomorphic HEMTs. Both types include a drain terminal, a source terminal and a gate terminal in which a voltage potential is applied to the gate terminal in order to control the electron flow within an undoped conductive channel layer between the source terminal and the drain terminal, in a manner that is well understood in the art. The conductive channel layer creates a potential well as a result of the disparities between the conduction band of the channel layer and the conduction band of the layers surrounding the channel layer. The difference between the regular HEMT and the pseudomorphic HEMT is that the pseudomorphic HEMT has a heterojunction in which the channel layer includes different semiconductor materials where the lattice constant of one semiconductor material is significantly different than the lattice constant of other semiconductor materials in the layer.
Current pseudomorphic HEMTs are generally aluminum gallium arsenide/indium gallium arsenide (AlGaAs/InGaAs) heterojunction devices that include a strained InGaAs channel that achieve improved device performance over regular HEMTs. High device performance allows the HEMT to handle larger amounts of current flow, and thus, higher power at higher frequencies. For an HEMT of this type, the lattice constants between the indium and the gallium arsenide is significantly different because the indium atoms are larger than the gallium and arsenide atoms. During crystalline fabrication of the device, the larger indium atoms create stresses in the crystalline structure which produce a strain in the channel layer. The thicker the channel layer, or the greater the concentration of indium atoms in the channel layer, the greater the strain. When the thickness of the channel layer reaches a "critical thickness" at a particular indium concentration, the strain in the channel layer becomes large enough that the channel layer relaxes (relieves stress), and dislocations in the channel layer are formed. These dislocations create faults in the crystalline lattice of the channel layer that have been known to affect device performance. Therefore, this built-in strain has been known to limit the usable width of the InGaAs channel layer to widths below the "critical thickness" to prevent strain relaxation that form dislocations.
The maximum width of the InGaAs channel layer in prior art pseudomorphic HEMT devices has been typically less than 150 .ANG.. A channel layer width this narrow has resulted in confined energy levels for electron transport that are relatively far from the bottom of the conduction band within the potential well formed by the InGaAs channel layer. This results in electron transport between the source terminal and the drain terminal at energy levels close to the top of the well formed in the channel layer. These high electron transport energy levels result in degradation of the confining properties of the conduction band at the channel edges causing electrons to scatter out of the channel layer. As electrons are scattered to higher energy levels during operation of the HEMT at high bias, the probability of the electrons being scattered into surrounding layers defining the InGaAs channel increases. The resulting electron transport in the surrounding layers outside of the InGaAs channel layer yields parallel transport paths that degrade device performance as a result of lower efficiency, lower transconductance, and lower overall RF performance. This is particularly true for high power devices with a high electron concentration in the InGaAs channel layer.
U.S. Pat. No. 5,060,030 issued to Hoke, Oct. 22, 1991, provides a detailed background discussion of the creation and effect of a strained InGaAs channel layer in a pseudomorphic HEMT. Hoke realizes and discusses the need to maintain the thickness of the strained channel layer below the "critical thickness". Hoke attempts to increase the channel thickness so as to increase the device performance without exceeding the critical thickness of the channel layer. To accomplish this, Hoke proposes providing a strained compensation layer made of a material, for example, boron gallium arsenide, to alleviate at least some of the strain in the channel layer so as to increase its critical thickness. As discussed in Hoke, the strain compensation layer increases the critical thickness of the channel layer by providing an intrinsic compressive stress which compensates for the intrinsic tensile stress of the channel layer.
Hoke offers one solution to increase the thickness of the InGaAs channel layer in a pseudomorphic HEMT device. However, the Hoke solution requires the incorporation of an additional layer, the strain compensation layer, that adds to device complexity and fabrication complexity. Further, the Hoke solution does not provide increase device performance at known channel layer thicknesses.
What is needed is a pseudomorphic HEMT having an InGaAs channel layer that has a thickness greater than the critical thickness of known strained HEMTs so as to lower the confining energy levels in the channel layer, without degrading the HEMT performance. It is therefore an object of the present invention to provide such an HEMT.